The present invention relates to a developing device for visualizing an electrostatic latent image on an electrostatic latent image retaining member by attaching toner thereto, and more particularly to a developing device of the noncontact type in which a developing process is carried out using a two-component developer consisting of a mixture of nonmagnetic toner and magnetic carrier in a state that developer is not in contact with the electrostatic latent image retaining member.
An image forming apparatus, operable in both a black/white mode and a full color mode, is known in which, for developing a full color image, toner images of different colors are successively superposed on an electrostatic latent image retaining member, and the resultant color toner image is transferred on a paper through one-time image transfer. In this type of the image forming apparatus, a transfer drum with a recording paper held thereon, which receives, every color, toner images from the electrostatic latent image retaining member, is not used. Accordingly, this type of the image forming apparatus is advantageous in that the size and cost reduction is possible. For the developing device in use with such an image forming apparatus, there is proposed a developing device of the noncontact type in which a developer carrying member is disposed at a location where it faces an electrostatic latent image retaining member with a gap therebetween so that a toner image formed on the electrostatic latent image retaining member is not disarranged in a subsequent developing process. In the developing process, the developer on the developer carrying member is not brought into contact with the electrostatic latent image retaining member.
The developing device of the noncontact type comes in two categories: a developing device of one-component type, and a developing device of two-component type. In the first developing device, for the development, toner alone is magnetically or electrostatically held on the developer carrying member. In the second developing device, for the development, a two-component developer consisting of nonmagnetic toner and magnetic carrier is magnetically attracted to the developer carrying member. The developing device of the two-component type in which a two-component developer is used, an alternating electric field is generated in a developing area where the developer carrying member faces the electrostatic latent image retaining member, and the nonmagnetic toner is caused to fly by electric field, is superior to the developing device of the one-component type in image quality, image sustaining performance, and others.
Published Unexamined Japanese Patent Application No. Sho. 56-144452 (Japanese Patent Publication No. Hei. 2-4908) proposes two types of developing device for the developing device of the two-component and noncontact type. In the first developing device, the developer carrying member contains a magnetic pole, which is fixed at a location facing the developing area. A conductive sleeve, which is provided on the outer surface of the developer carrying member, is turned during the developing process. Thus, the developing process is carried out in an alternating electric field. In the second developing device, the developer carrying member contains two magnetic poles of the same polarity arranged juxtaposed. The developer carrying member is disposed such that a portion thereof between the magnetic poles faces the electrostatic latent image retaining member. With this arrangement, toner is easy to separate from the carriers during the developing process.
In the developing device of such a noncontact type, to obtain a sufficient development density, an intensive electric field must be formed in the developing area. To realize this, the conductive sleeve serving as an electrode must be put close to the surface of the developer carrying member within the limits of the possible. As described above, the first developing device locates one magnetic pole in the developing area. The second developing device juxtaposes two magnetic poles of the same polarity in the developing area so as to form two maximum values of the magnetic field of which the direction is vertical to the sleeve surface, and one minimum value of the magnetic field located between the two maximum values. Therefore, the developer is erected high in the developing area. As a result, it is difficult to keep the developer separated from the developer carrying member and to secure a density necessary for the development.
In view of this circumstance, proposals, which can keep the developer separated from the developer carrying member and secure the smallest gap between the electrostatic latent image retaining member and the sleeve, have been made in Published Unexamined Japanese Patent Application Nos. Hei. 1-291268, Sho. 60-176069 (Japanese Patent Publication No. 4-36383), and sho. 63-159868. The developing device described in Published Unexamined Japanese Patent Application No. Hei. 1-291268 proposes some unique techniques as follows. The pitch of the array of the magnetic poles in the developer carrying member is reduced. Saturation magnetization of the magnetic carriers of the two-component developer is reduced. In the electrostatic latent image retaining member, the circumference repulsion magnetic pole is disposed in opposition to the developing magnetic poles in the developer carrying member. With provision of the magnetic pole, the erection of the developer on the developer carrying member is reduced in height.
In the developing device disclosed in Published Unexamined Japanese Patent Application No. Sho. 60-176069 (Japanese Patent Publication No. 4-36383), a plural number of magnetic poles are disposed at such locations off the developing area in the developer carrying member. Magnetic poles of different polarities are disposed and fixed before and after the developing area. The developing process is carried out in a noncontact manner while only the sleeve on the outer surface of the developer carrying member is turned.
In the developing device disclosed in Published Unexamined Japanese Patent Application No. Sho. 63-159868, a magnetic field generating means is located within the developer carrying member. A magnetic field developed by the magnetic field generating means is shaped so that the magnetic field in the tangential direction on the surface of the developer carrying member has two maximum values of its intensity on both sides of at a portion where the developer carrying member is closest to the electrostatic latent image retaining member, and the minimum value of the intensity of the magnetic field in the tangential direction in the vicinity of the closest portion is at least 90% of the maximum value.
The developing devices disclosed in the above-mentioned publications have the problems to be described below.
The developing device described in Published Unexamined Japanese Patent Application No. Hei. 1-291268 succeeds in preventing the magnetic carriers from running in a line in the direction orthogonal to the surface of the developer carrying member, and hence in lessening the erection of the developer. Through the interaction of the magnetic fields developed by the repulsion magnetic poles of the electrostatic latent image retaining member and the developing magnetic poles, the erected developer lies down on the circumference of the developer carrying member. As a result, a portion where the developer is erected toward the electrostatic latent image retaining member is reduced. Toner issues mainly from the portion where the developer is erected toward the electrostatic latent image retaining member. Accordingly, if such a portion of the developer is reduced, a less amount of toner issues, so that the developing efficiency is reduced. For the magnetic pole pitches in the developer carrying member, the publications merely describes that the magnetic pole pitch of the second developing device group is shorter than that of the first developing device group, and the magnetic pole pitches of the third and subsequent developing devices are further shorter, and that, with the shortening of the magnetic pole pitch, the height of the erected developer can be reduced. And there is no description on the relationships of the magnetic pitches, the erection of the developer, and resultant developing characteristics.
In the developing device disclosed in Published Unexamined Japanese Patent Application No. Sho. 60-176069 (Japanese Patent Publication No. 4-36383), under the influence of the magnetic fields developed by the different magnetic poles disposed off the developing area, the magnetic carriers are attracted onto the developer carrying member in a state that it continuously runs on the circumference of the developer carrying member. The erected developer lies down. In this state, of the toner attracted to the erected developer, only the toner attracted to the surfaces of the erected developer, which face the electrostatic latent image retaining member, flies into the developing area, and contributes to the development. The resultant development density is unsatisfactory.
In the developing device disclosed in Published Unexamined Japanese Patent Application No. Sho. 63-159868, the force of restraining the magnetic carriers is reduced in the developing area. The erected developer lies down on the circumference of the developer carrying member. A less amount of toner contributes to the development, and a development density is insufficient.
With view of overcoming the disadvantages of the conventional art as mentioned above, the present invention has an object to provide a developing device of the noncontact type which reproduces an image at sufficient development density by using a two-component developer.